Solvent recovery



Aug. 8, 1939. w. w. KRAFT SOLVENT RECOVERY Original Filed May 3l, 1935 HMM. lindm Patented Aug. 8, 1939 soLvEN'r ancovalw Wheaton W. Kraft, New York, N. Y., as'slgnor to The Lummus Company, New York, N. Y., a

corporation oi Delaware Application May 31, lass, serial No. 24,308

RenewedFebruai-y 13, i939 5 Claims. (on. 20a-se) The present invention relates to solvent recovery and more particularly to methods and apparatus for recovering solvent following a solvent extraction treatment of petroleum.

In solvent extraction of petroleum, a wide variety of solvents. may beemployed, but in any case, the end products comprise two mixtures: `first, a solvent-extract mixtureconsisting'of a large proportion of solvent with a small amount of extracted oils of generally naphthenic characteristics, and second, a. solvent-rafllnate mixture comprising the desired oils of.. paraflinic characteristics mixed with a relatively small amount of solvent. Owing to the large quantity of solvent with respect to the oils treated, it is necessary to effect substantially complete, recovery of the solvent. Furthermore, the proportion of water in the recovered solvent must be maintained below a certain limit of tolerance.

Water is introduced into the system in either of two ways: rst, from an external source as by the unavoidable presence of water in the oil, and second, by the use of stripping steam which is necessary or desirable for effecting the iinal stripping of solvent from the oils. Practically all solvents useful for extractionform azeotropic or constant boiling mixtures with water, and this fact introduces difiiculties all out of proportion to the small quantity of water involved. As a consequence, the solvent recovery plant has frequently been uneconomical, either because of the size and complexity of the apparatus or because of the necessity for wasting a part of the solvent.

The object of the present invention is to provide a continuous process for recovering the solvent from the extract and railinate and for providing a simple and effective system for obtaining the solvent free of Water to the required tolerance.

With this object in view, the principal feature of the invention consists in a series of steps whereby the water, whether introduced from an .external source or from the steam stripping operation, is concentrated in a relatively small quantity of the total solvent. The greater portion of the solvent is recovered dry or nearly so and only the relatively small quantity of solvent which is associated with water is treated forultimate separation. y

The accompanying drawing is a diagram of the preferred form of apparatus for practising the present invention.

As a result of the solvent extraction, which is carried out in the apparatus I, there are obtained two streams designated at 2 and d, TheV stream ,2 is asolvent-extract mixture which usually contains 90 vto 95%. of -solvent, possibly between 0.5 .and 1% of water, and the extracted naphthenic oils. 'Ihe stream 4 comprises solvent and rafflnate without water, the ramnate usually conls stituting at least of the mixture. The solvent may be any usual or preferred material such as phenol, -cresylic acld,.i'urfural, etc. 4 The vsolvent-extract stream 2 presents the greatest difficulties. because of the extremely large quantity of solvent with respect to extract and also because of the water present. Some of the water is carried with the solvent itself, since the recovery seeks to. reduce the water content only to a suitable tolerance (generally about 0.5%), and the remainder is accounted for by its presence in the oil, from small leaks in water cooled equipment or from accumulations formed by steaming of the equipment during ,a shutdown. -The solvent-extract stream is subjected to a preliminary distillation for removal of the major part of the solvent in vaporizing towers l and 8. The stream is heated in a heat exchanger Ill and discharged into the low pressure tower 6. The vapors thus formed may comprise i0 to 50% of the solvent of the original mixture, together with substantially all of the water. These vapors of solvent and water are removed from the tower t by a pipe l2 and are subjected to further treatment to be presently described. The residue from the primary vaporizing tower t is passed through a pipe still lll into the high pressure vaporizer 8, in which nearly all of the remaining solvent is vaporized, the vapors being passed to the heat exchanger lt and there condensed to obtain dry solvent condensate which is drawn off through the pipe lli. The residue from the column 8 comprises all of the original extract with only a small amount of solvent (about 1 to 5% of the solvent in the original mixture). This residue is withdrawn from the tower 8 by a pipe IB to be passed to equipment for substantially complete removal of the solvent from the extract.`

Returning now to the stream l2 comprising the 45 vapors of solvent with the externally introduced water, this stream is run to a partial condenser 2t in which there is condensed as much of the solvent as possible without condensing water to a higher degree than the limit of tolerance. The 50' condensed solvent` with the limiting quantity of water is withdrawn at 22 while the uncondensed vapors are passed by a pipe 24 to a dehydrating column 26. The pipe 24 contains all of the externally introduced water above the limit of tol- 66 erance and the amount of solvent associated therewith is only a fraction, usually not more than 25%, of the total amount of solvent in the whole system. The streams I6 and 22 which i'orm the bulk of the solvent are dry, or sufllciently so, and therefore present no problem of water removal. 'I'he streams I6 and 22 are run to a tank 28 which may be conveniently built in the bottom of the column 26.

The stream 24 accounts for the externally introduced Water, but the column 26 is also to handle a small solvent-water stream resulting from steam stripping of the solvent and oil mixtures.

The reduced solvent-extract mixture obtained at I8 is heated in a pipe stillv 28 and introduced into an atmospheric hash column 38 to obtain dry solvent vapors at 3l, the vapors being fractionated by solvent reilux'introducedfrom any convenient source at 32. The residue, which is now still further concentrated in extract is passed by a pipe 34 in'to a stripping column 36 which for convenience is made integral with the column 30. Steam is introduced into the bottom of the column, as indicated at 38, and the extract, denuded of solvent, is withdrawn at 40. The vapors, comprising the solvent with stripping steam, are removed from the top of the column at 42, the composition of the vapors being controlled by reflux introduced at 44. The vapor pipe 42 is run to the dehydrating column 26.

Referring now to the solvent-raillnate stream 4, this is heated in a pipe still 46 and introduced into an atmospheric flash column 48 and a steam stripping column 50. 'I'he columns 48 and 58 are similar to the columns 38 and 36 used for treating the reduced solvent-extract mixture. Dry solvent vapors are obtained from the top of the column 48 and are passed into a pipe 52 with which is joined the vapor pipe 3| from the column 30. Raflinate is withdrawn from the bottom of the stripping column at 64. Vapors of solvent and stripping steam are obtained from the top of the stripping column by the pipe 56, which joins the pipe 42 leading to the dehydrating column. The quantity of vapor introduced by the pipe 42 is small and ordinarily will contain considerably less solvent than the stream 24.

The dehydrating column 26 to which the streams 24` and 42 are passed comprises a fractionating column having decks 62 below and above the points of feed. The column has a pipe 63 leading to the tank 28. Since the total amount of water introduced at 24 and 42 is small, the total feed to the column is materially on the solvent side of the constant boiling mixture and therefore there occurs in the column a fractionation between solvent as residue andthe constant boiling mixture of solvent and water as distillate. The column operates on the heat contained in the entering streams, particularly the vapor stream 24, which is large compared with the small volume of distillate evolved. To augment the heat supply, the dry solvent vapor stream 52 from the flash columns 30 and 48 is run to the bottom of the dehydrating column. The operation of the column by the heat of the vapor streams, and without additional heat, forms a feature of the invention.

The vapors of the constant boiling mixture pass from the top of the column by a pipe 66 to a partial condenser 68 from which the condensate is returned to the column as reflux by a pipe 18. The uncondensed vapors of the constant boiling mixture are removed from the condenser at 12.

The constant boiling mixture at 12 contains all of the water above the limit of tolerance, which has been introduced into the system either from external sources or as stripping steam. The water introduced from external sources ls extremely small, and neglecting the tolerable limit of water recycled with the solvent, it is rarely more than a small fraction of one percent of the total amount of solvent in the whole system. Moreover, the amount of stripping steam is also small, since it is used only to remove the last traces of solvent in th columns 36 and 60. As a consequence, the total quantity of constant boiling mixture at 12 may be of the order of magnitude of 1 or 2% of the total quantity of solvent, while the quantity of solvent contained in the mixture forms an even smaller proportion, depending on its characteristics. For example, cresylic acid forms only about 6% of the constant boiling mixture, while furfural would make up about 35% of the mixture. In any case, the difculties introduced by the presence of water have been localized in an extremely small part of the whole system.

The quantity of solvent contained in the constant boiling mixture, although small, is not negligible, and its continuous loss could not be tolerated. The constant boiling mixture may be handled in any suitable manner, for example, by passing it to a total -condenser 14 wherein it is condensed and cooled to a temperature to permit it to settle in two layers in a vessel 16. The lower or solvent layer, containing a higher proportion of solvent than the constant boiling mixture, is returned to the dehydrating column by a pipe 18, while the upper or water layer, containing a higher proportion of water than the constant boiling mixture, is passed to a water removal column 18, wherein the material is subjected to fractionation between water as residue and the constant boiling mixture as distillate. Water is withdrawn at 80 and the constant boiling vapor mixture is run by a pipe 82 to the total condenser 14. This system provides for continuous breaking up of the mixture into solvent and Water. It is to be understood, however, that the precise handling of the constant boiling mixture does not form a part of the present invention and any other suitable method may be employed, as desired. For example, with certain solvents, the invention described in the Kraft and Zimmerman application Serial No. 19,245, filed May 1, 1935, may be employed, by which the layers resulting from the condensation and cooling of the constant boiling mixture may be directly re-used in the process It will be seen that the present invention provides (a) for handling of the externally introduced water by localization of the water in a stream of solvent which is of small quantity compared with the total solvent used in the system; (b) for such a distillation procedure that the stripping steam is small in quantity and is associated with only a small amount of solvent; and (c) for ultimate concentration and localization of all water above the limit of tolerance in a small quantity of constant boiling mixture which may be conveniently handled separately from the bulk of dry solvent.

Having thus described the invention, what is claimed is:

l. In solvent recovery following a solvent extraction treatment of petroleum oils with a solvent of a character to form with water an azeotropic mixture of minimum boiling point, the

amano method which consistsin distilling from a solvent-extract mixture having a small amount of water unavoidably contained therein a solventsolvent-water distillate, fractionating said solvent-water distillates substantially between solvent and the azeotrope, thereby to localize the water, including that initiallyv introduced and that used as strippingsteam,A in association` with a quantity of solvent which is small in comparison with the total solvent used in the system, and finally separating'the azeotropic mixture-to recover substantially dry solvent therefrom.

2. In solvent recovery following a solvent extraction treatment of petroleum oils' with a solvent of a character t9 form with water an azeotropic mixture of minimum boiling point, the method which consists in distilling from a solvent-extract mixture having a small amount of water unavoidably contained therein a solventwater distillate containing a part of the solvent and substantially all of the water, leaving a solvent-extract residue with no water in excess of a predetermined limit of tolerance, subjecting said residue to dry distillation to obtain the greater part of the solvent as dry vapors and to 1 leave a residue containing only a small proportion' of solvent, ystripping the solvent from the i last-named residue with steam to form a second solvent-water distillate, passing the solventwater distillates toa fractionating zone to fractionate substantially between solvent and the azeotropic mixture. introducing dry/solvent vapors at the bottom of the fractionating zone to supply heatforthe fractionatingv operation, and nnallynseparatlng the azeotropic mixture to recover dry solvent therefrom.

3. In solvent recovery following a solvent extraction treatment of petroleum oils with a solvent of a character to form with water an azeotropic mixture oi minimum boiling point, the method which consists in distilling from a solvent-extract mixture having a small amount of y water unavoldably contained therein solventwater vapors containing apart of the solvent mi4 i vent-extract residue with no water in excess of and substantially all of the water, leaving a sola predetermined limit of tolerance, condensing from the vapors as much solvent as possible without condensing water to exceed the limit of tolerance, and passing uncondensed vapors to -ating the vapors in said zone substantially between solvent as residue and the azeotrope as distillate, whereby all of the water is localized in association with a relatively small quantity of solvent, and iinally separating the azeotropic mixture to recover substantially dry solvent therefrom. 1 i 1 i il "fi 4. In solvent recovery following a solvent extraction treatment of petroleum oils-with a solvent of a character to form with water an azeotropic mixture of minimum boiling point, the method which consists in subjecting a solventextract mixture having a small amount of water unavoidably contained therein to distilling steps to obtain a solvent-water distillate containing a part of the solvent and substantially all of the water, a dry solvent distillate comprising the greater part of the solvent in the system, and a solvent-extract residue containing only-a small proportion of solvent, stripping the solvent from the last-named residue with steam to form a second solvent-water distillate, fracticnating said solvent-waterl distillates substantially between solvent and the azeotrope, thereby to localize the water, including that initially introduced and that used as stripping steam. in association with a quantity of solvent which is small in comparison with the totalsolvent used in .the system, and finally separating the azeotroplc mixture to recover substantially dry solvent therefrom.

5. In solvent recovery following a solvent extraction treatment of petroleum oils with a solvent of a character to form with water an azeotropic mixture of minimum boiling point, Vthe method which consists in subjecting a solventextract mixture having a small amount of water unavoidably contained therein to distilling steps to obtain a 'solvent-water distillate containing a part of the solvent and substantially all of the water, a dry solvent distillate comprising the greater part of the solvent inthe system, and a solvent-extract residue containing only a small proportion of solvent, stripping the solvent from the last-named residue with steam to form a Kam. sol 

